Combustion of fuels such as wood, coal, etc. and incineration of wastes such as municipal wastes, industrial wastes, etc. is a common practice to reduce the amounts of waste and to recover its energy content. Incineration results in production of by-products such as ashes (bottom ash and fly ash) and by-products from gas cleaning operations.
Air pollution control (APC) by-products originate from the treatment of the gas and particles coming out from the incineration chamber. Examples for treatment operations for air pollution control include electrostatic filters for separation of particulate impurities often called fly ash, dry scrubbers for separation of gaseous impurities such as hydrogen chloride, hydrogen fluoride, sulphur dioxide, etc. e.g. by reaction with lime, wet scrubbers for removal of gaseous impurities by scrubbing in an aqueous solution such as an acid, neutral solution or base, and treatment of aqueous effluents generated from scrubbing operations and condensation operations by various wastewater treatment technologies.
Air pollution control (APC) by-products are materials composed of primarily inorganic compounds. The major components are generally Ca, Si, Cl, K, Na, Al, Zn, and Pb. APC by-produces contain also other elements such as iron, phosphorus and a range of heavy metals such as Ti, Cd, Ni, Cr, Cu, B, etc.
APC by-products have generally a very high content of water-soluble chloride salts which makes disposal of APC by-products difficult and costly. In Sweden, waste material for disposal is generally classified according to four categories regarding its water-soluble chloride content as measured by standard leaching tests: inert waste (<800 mg Cl per kg dry matter), non-hazardous waste (800-15 000 mg Cl per kg dry matter), hazardous waste (15 000-25 000 mg Cl per kg dry matter), and material exceeding the criteria for hazardous waste (>25 000 mg Cl per kg dry matter).
In general, the content of water soluble chlorides in APC by-products is in the order of 50 000 to 200 000 mg Cl per kg dry matter, thus exceeding the criteria for hazardous waste disposal. Since disposal is prohibited, one common practice is to export the APC by-products and dispose it in special old mines.
The problems of disposing APC by-products have led to the development of methods to wash APC by-products from their soluble chloride content to enable disposal of the washed material as non-hazardous waste. The relatively high content of zinc in APC by-products has also led to development of methods of combining washing of APC by-products to reduce chloride content with zinc recovery. Such processes are based on washing of APC by-products with an acid to dissolve their zinc content followed by recovery of zinc by precipitation as hydroxides or recovery of zinc in elemental form by using a combination of solvent extraction and electroplating on cathodes.
Washing of APC by-products with water or acid results in a wastewater containing a mixture of chloride salts together with other impurities such as heavy metals, etc. The wastewater has often to be treated for removal of impurities such as heavy metals, sulfate, ammonia, etc. before being discharged into a recipient.
Chloride is a water-soluble anion which is not precipitated during zinc recovery as zinc hydroxide, and not extracted by solvents used for zinc extraction for production of elemental zinc by electroplating. Chloride is also not removed from solution by chemical treatment in wastewater treatment plants. The effluent from washing of APC by-products is thus saline and in many cases exceeds the limits for being discharged to a municipal wastewater treatment plant due to corrosion aspects and toxic effect on bacteria in the biological treatment plant. The common practice is to discharge such effluent directly into the sea.
However, in many cases landfills or incinerators are located far from the sea in locations in which there are strict limits regarding discharge of chlorides to a recipient. In these locations it is not possible to wash or recover zinc from APC by-products without taking care of the soluble chloride salts since it is prohibited to discharge the saline effluent.
It is known in the art to treat problematic effluents with so called zero liquid discharge systems (ZLD). In these systems a saline effluent is treated usually by vacuum evaporation to reduce the amount of generated waste and to recover a purified condensate which can be used as potable water or being discharged to a recipient.
Wash-water from treatment of APC by-products can be treated with a ZLD system instead of treatment in a wastewater treatment plant with subsequent discharge of saline effluent. In this way discharge of a saline effluent can be avoided. Use of ZLD system for treating saline effluents such as wastewater from flue gas cleaning operations is becoming more and more common. ZLD systems are also used for treatment of saline effluents not originating in incineration operations. Examples include, wastewater from shale gas fracking operation, RO desalination concentrates, ion exchange softeners concentrate, landfill leachate, mine waters, etc.
A main disadvantage of zero liquid discharge systems is production of a by-product salt mixture. The by-product salts of ZLD systems are usually hygroscopic in nature, i.e. it's a moist paste of a water-soluble salt mixture with no end use. Such by-products cannot be disposed in a landfill as hazardous waste due to risk for leaching of water-soluble salts. The by-product waste must usually be disposed at special locations e.g. in salt mines.
Potassium is a resource which is mainly used as a raw material for production of fertilizers. Today potassium is extracted from limited rock deposits of minerals such as syvine (KCl), kainite (KCl.MgSO4.3H2O), carnallite (KCl.MgCl.6H2O) usually together with kainitite (NaCl) by conventional mining or by solution mining, i.e. dissolving the rock with a solution that can be pumped from the mine to the processing plant. Potassium is also recovered from salt lakes usually by natural evaporation forming crystallization of a mixture of carnallite and kainitite which is thereafter processed for separation and recovery of potassium chloride. Potassium resources are limited and the major part of the currently known world's reserves are located in only two countries Canada and Russia. Therefore, there is a general environmental interest in recovering potassium salts from wastes such as from APC by-products in order to increase the life time of non-renewable limited rock deposits.
In general, there is a strong interest in society to convert wastes into products thus to minimize the need for mining natural resources. Such approaches contribute to reduce the negative environmental effects associated with mining of natural resources. Furthermore, there is a general environmental interest to convert wastes into products in order to minimize disposal of wastes with associated negative environmental impact and to obtain other benefits such as reduction of energy use due to recycling, etc.
The saline leach solution from washing of APC by-products typically comprises of a mixture between calcium, sodium, and potassium chloride salts. However, the relative amounts of the different salts differ from time to time and from plant to plant. Especially if APC by-products originating from several incinerators are to be washed in a single central plant.
The U.S. Pat. No. 6,319,482 discloses treatment of fly ash/APC residues including lead salt recovery. The fly ash/APC residues have a high content of CaCl2, which is of interest for recovery. In a first stage, fly ash/APC residues are washed followed by a phase separation to obtain a first calcium enriched filter and a filtrate. In a second step, the filtrate is processed for metal recovery by pH adjustment. In a third stage, the remaining brine is concentrated by evaporation to recover a concentrated and purified calcium chloride brine. Precipitated mixture of KCl and NaCl is removed as a side product and dumped as disposal or used as road salt. The main goal of the process is to recover CaCl2, which unfortunately leave the disposal of the NaCl and KCl mix essentially unaddressed. The use of a mix of NaCl and KCl as road salt is furthermore not optimal, since the KCl content mainly contributes to fertigation of the environment but is not very efficient for reduction of the water melting point. The focus on recovery of CaCl2 also results in that the process is only operable on initial materials having a high CaCl2 content.
In the publication “Recovery of soluble chloride salts from the wastewater generated during the washing process of municipal solid wastes incineration fly ash” by H. Tang et al in Environmental Technology, 2014, vol. 35, No. 22, pp 2863-2869, recovery of chloride salts from municipal waste incineration fly ash is discussed. A method to separate the three salts is suggested. The wash solution is evaporated in three different evaporators to obtain three different fractions of crystallized salts. In a first fraction, almost pure NaCl is obtained. In a second fraction, a mix between NaCl and KCl is obtained. After dissolving this fraction into water, ethanol is added, resulting in precipitation of pure KCl. A third fraction comprises the remaining NaCl together with some KCl and CaCl2, leaving a solution of only KCl and CaCl2. The third fraction is returned to the start solution, while the KCl and CaCl2 in the solution are separated by addition of ethanol, precipitating KCl and leaving the CaCl2 in the final solution. The process is complex and high in capital costs due to the need for three evaporators. If the ethanol is to be reused, distillation steps have to be performed, making the process complex and energy-consuming.
In the published Japanese application JP 2011-14846A, recovery of KCl from municipal waste incineration fly ash is disclosed. During the washing of the ash the CaCl2 is caused to precipitate into the remaining ash by addition of carbon dioxide, forming CaCO3. The KCl is separated at a low temperature <20° C. A main disadvantage of this process is requirement for a large amount of carbon dioxide. Furthermore, since conversion of CaCl2 to CaCO3 requires balancing the released Cl anion with a cation, large amount of base such as NaOH is usually required in order for the process to obtain reasonable yield of CaCl2 conversion. In addition, CaCl2 is not recovered in the process, instead it is transformed into CaCO3 in the fly ash increasing the amounts required to be disposed and associated costs considerably.
U.S. Pat. No. 2,839,360 discloses a method for reducing the concentration of alkali metal salts in calcium chloride brines by forming a double salt between KCl and CaCl2. U.S. Pat. Nos. 3,279,897 and 3,212,863 disclose a method for precipitating KCl from salt mixtures by addition of ammonia. U.S. Pat. No. 3,359,079 discloses a method for precipitation of potassium halides from mixed brines using organic solvents. Use of ammonia or organic solvents for recovery of potassium chloride requires complex distillation systems and usually also several evaporators which makes the process complex and energy consuming.
There is a need for a method that can enable to separate and recover chloride salts form APC by-products, ZLD by-products and saline effluents in form of commercial products (with possibility for different levels of purity) for sale. Such an approach will solve one of the disposal problems of APC by-products independent on the location for operation. Furthermore, the costly disposal of ZLD by-products will be omitted.
In Sweden it is common to use sodium chloride for deicing of roads. The use of road salt in Sweden is about 300 000 tons per annum. It is common to spread sodium chloride as a 23% by weight solution. Spreading sodium chloride in a liquid form has the advantage of fast reaction compared to spreading solid sodium chloride, this since solid sodium chloride needs to absorb heat from the environment in order to dissolve into a solution which can act as a deicer.
Getting the road to dry up as quickly as possible is important in connection with deicing. The risk of re-freezing minimizes when the road surface becomes dry. The number of road salt spreading operations become fewer which enables saving on both fuel and chemicals. Use of sodium chloride as a road salt is generally known to cause the road to dry up fast. In contrast, by using calcium chloride as a road salt, the roads does not dry as fast since calcium chloride is an hydroscopic salt which keeps moisture in the roadway. Therefore, use of pure calcium chloride as a road salt for deicing purposes has a significant disadvantage compared to use of sodium chloride.
Using pure sodium chloride as a road salt has also disadvantages. The main disadvantage of pure sodium chloride is that its effect decreases when the temperature drops down to the range of about −5 to −7° C., and has no effect at all below −9° C. In contrast, calcium chloride is effective down to −20° C. (Theoretically, under ideal conditions down to −40° C.).
There is a need for a simple process for separating and recovering CaCl2, NaCl and KCl in essentially pure forms from their mixtures. There is also a need for a process enabling separation of three salts without a need for distillation of organic solvents or multiple evaporation steps. There is further a need for a robust salt recovery process that can handle the large variability in the ratios of the salts in the feed over time since composition and ratios of elements in wastes vary very much over time and total amounts are much smaller in comparison to mineral reserves or salt lakes. There is an additional need for a salt recovery process from washing APC by-products to enable operation in locations in which discharge of saline effluents is prohibited. There is a need for a process that can enable processing saline effluents without generation of problematic by-product wastes. There is a need for a process that can enable to reduce chloride content in APC by-products to enable disposal in landfill and at the same time minimize the weight of material being disposed. There is also a need for a process that can enable to recover potassium from APC/ZLD by-products in a pure form suitable for use as a raw material for e.g. fertilizers. There is an additional need for a process that can enable to recover sodium chloride from APC/ZLD by-products in a pure form suitable for use as road salt or a raw material for e.g. chlor-alkali industry. There is further a need for a process that can enable to recover calcium chloride from fly ash APC/ZLD by-products in a pure form suitable for use as e.g. deicing or dust control material or as a raw material for industrial processes. There is a general need for processes that enable to convert wastes into products.